1. Field of the Invention
The present invention relates to a secondary air feed device of an engine.
2. Description of the Related Art
In a known engine, to promote the oxidization of unburned HC and CO in the exhaust gas, and thus purify the exhaust gas, secondary air is fed into the exhaust passage of the engine. In this engine, the target amounts of secondary air, which are a function of the engine load and the engine speed, are memorized in advance, and the feeding of secondary air into the exhaust passage is carried out on the basis of the target amounts of secondary air. Further, an oxygen concentration detector is arranged in the exhaust passage, and the amount of secondary air is corrected by a feedback operation, so that the concentration of oxygen in the exhaust passage becomes equal to a predetermined concentration (see Japanese Unexamined Patent Publication No. 59-138714).
Namely, even where the target amounts of secondary air are memorized in advance as a function of the engine load and the engine speed, and the amount of secondary air is determined so that it becomes equal to the target amount, since irregularities in size and shape of parts of the secondary air feeding device exist, the amount of secondary air actually fed into the exhaust passage considerably deviates from the target amount. Particularly when the engine has been used for a long time, and thus an aging deterioration (wear) of the parts has occurred, the amount of such a deviation becomes very large. Accordingly, in this engine, as mentioned above, to correct the deviation of the amount of secondary air actually fed into the exhaust passage, relative to the target amounts of secondary air, the amount of secondary air is feedback-controlled based on the output signal of the oxygen concentration detector.
In such a feedback control, however, the amount of secondary air actually fed is controlled after it has deviated from the target amounts of secondary air, and in addition, since a time delay exists in the secondary air control device, even if the feedback control signal indicating that the amount of secondary air is to be changed is input to the secondary air control device, some time must elapse before the amount of actually fed secondary air is changed. Accordingly, a problem arises in that, even if the amount of secondary air is controlled by such a feedback control operation, it is difficult to maintain the concentration of oxygen in the exhaust passage at a constant value, particularly when the required amount of secondary air is abruptly changed, and further, another problem arises in that the oxygen concentration detector must be arranged in the exhaust passage, to carry out such a feedback control operation.